Transcrystallinity may occur during differential scanning calorimetry analysis at the surfaces of the samples. In such a case, measurements may be unsuitable. We propose simple methods for the determination of intrinsic crystallization data that are accurate for the polymer and for the determination of the nucleating ability of the surfaces. These methods are based on the experimental analysis of the crystallization of samples with different and calibrated thicknesses during experiments at different constant cooling rates. Analysis of thin samples allowed the characterization of transcrystallinity, whereas analysis of at least three samples of different thicknesses allowed us to determine the true crystallization kinetics of the bulk material. These two techniques were independent of each other and were successfully applied to the case of a high-density polyethylene. The determinations were verified with simple analytical models. A further extension could be the study of the nucleating ability of different substrates.